When a magnetic disk is rotated at a high speed, a lubricant of a lubrication film formed on a surface of the magnetic disk tends to be splashed easily by a shearing force caused by air and a centrifugal force thereof.
As prior art for increasing molecular weights of lubricants, methods for removing low molecular weight components and impurities by solvent extraction are disclosed in JP No.267707, JP-A Nos. 4-31005, 5-20673, 5-234066, 5-109053, 6-215347, 7-141644, 9-291296, 9-157674, and 10-53781.
Lubricants that have been polymerized to have a high molecular weight by the solvent extraction method described in JP No.2677074, however, have such a problem that a substitution ratio of its end-group's function group drops depending on its type of lubricants, and as a result, a quantity of splash of its lubricants cannot be reduced.
Further, in order to enable a molecular weight control of a substantial amount of lubricants for industrial use by means of a splitter liquid chromatography, gel permeation chromatography, or evaporation method, provision of large-scaled facilities is required, which is economically disadvantageous.